/**
 * Copyright 2012-2013 eBay Software Foundation, All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
 * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations under the License.
 */
package com.erudika.para.server.utils.filters;

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.lang3.StringUtils;

/**
 * <p>
 * A {@link Filter} that enable client-side cross-origin requests by implementing W3C's CORS (<b>C</b>ross-<b>O</b>rigin
 * <b>R</b>esource
 * <b>S</b>haring) specification for resources. Each {@link HttpServletRequest} request is inspected as per
 * specification, and appropriate response headers are added to {@link HttpServletResponse}.
 * </p>
 *
 * By default, it also sets following request attributes, that helps to determine nature of request downstream.
 * <ul>
 * <li><b>cors.isCorsRequest:</b> Flag to determine if request is a CORS request. Set to <code>true</code> if CORS
 * request; <code>false</code> otherwise.</li>
 * <li><b>cors.request.origin:</b> The Origin URL.</li>
 * <li><b>cors.request.type:</b> Type of request. Values: <code>simple</code> or <code>preflight</code> or
 * <code>not_cors</code> or <code>invalid_cors</code></li>
 * <li><b>cors.request.headers:</b> Request headers sent as 'Access-Control-Request-Headers' header, for pre-flight
 * request.</li>
 * </ul>
 *
 * @author Mohit Soni
 * @see <a href="http://www.w3.org/TR/cors/">CORS specification</a>
 *
 */
public final class CORSFilter implements Filter {

	// ----------------------------------------------------- Instance variables

	/**
	 * Holds filter configuration.
	 */
	private FilterConfig filterConfig;

	/**
	 * A {@link Collection} of origins consisting of zero or more origins that are allowed access to the resource.
	 */
	private final Collection<String> allowedOrigins;

	/**
	 * Determines if any origin is allowed to make request.
	 */
	private boolean anyOriginAllowed;

	/**
	 * A {@link Collection} of methods consisting of zero or more methods that are supported by the resource.
	 */
	private final Collection<String> allowedHttpMethods;

	/**
	 * A {@link Collection} of headers consisting of zero or more header field names that are supported by the resource.
	 */
	private final Collection<String> allowedHttpHeaders;

	/**
	 * A {@link Collection} of exposed headers consisting of zero or more header field names of headers other than the
	 * simple response headers that the resource might use and can be exposed.
	 */
	private final Collection<String> exposedHeaders;

	/**
	 * A supports credentials flag that indicates whether the resource supports user credentials in the request. It is
	 * true when the resource does and false otherwise.
	 */
	private boolean supportsCredentials;

	/**
	 * Indicates (in seconds) how long the results of a pre-flight request can be cached in a pre-flight result cache.
	 */
	private long preflightMaxAge;

	/**
	 * Controls access log logging.
	 */
	private boolean loggingEnabled;

	/**
	 * Determines if the request should be decorated or not.
	 */
	private boolean decorateRequest;

	// --------------------------------------------------------- Constructor(s)
	/**
	 * No-args constructor.
	 */
	public CORSFilter() {
		this.allowedOrigins = new HashSet<>();
		this.allowedHttpMethods = new HashSet<>();
		this.allowedHttpHeaders = new HashSet<>();
		this.exposedHeaders = new HashSet<>();
	}

	// --------------------------------------------------------- Public methods
	@Override
	public void doFilter(final ServletRequest servletRequest,
			final ServletResponse servletResponse,
			final FilterChain filterChain) throws IOException,
			ServletException {
		if (!(servletRequest instanceof HttpServletRequest)
				|| !(servletResponse instanceof HttpServletResponse)) {
			String message
					= "CORS doesn't support non-HTTP request or response.";
			throw new ServletException(message);
		}

		// Safe to downcast at this point.
		HttpServletRequest request = (HttpServletRequest) servletRequest;
		HttpServletResponse response = (HttpServletResponse) servletResponse;

		// Determines the CORS request type.
		CORSFilter.CORSRequestType requestType = checkRequestType(request);

		// Adds CORS specific attributes to request.
		if (decorateRequest) {
			CORSFilter.decorateCORSProperties(request, requestType);
		}
		switch (requestType) {
			case SIMPLE:
				// Handles a Simple CORS request.
				this.handleSimpleCORS(request, response, filterChain);
				break;
			case ACTUAL:
				// Handles an Actual CORS request.
				this.handleSimpleCORS(request, response, filterChain);
				break;
			case PRE_FLIGHT:
				// Handles a Pre-flight CORS request.
				this.handlePreflightCORS(request, response, filterChain);
				break;
			case NOT_CORS:
				// Handles a Normal request that is not a cross-origin request.
				this.handleNonCORS(request, response, filterChain);
				break;
			default:
				// Handles a CORS request that violates specification.
				this.handleInvalidCORS(request, response, filterChain);
				break;
		}
	}

	@Override
	public void init(final FilterConfig filterConfig) throws ServletException {
		// Initialize defaults
		parseAndStore(DEFAULT_ALLOWED_ORIGINS, DEFAULT_ALLOWED_HTTP_METHODS,
				DEFAULT_ALLOWED_HTTP_HEADERS, DEFAULT_EXPOSED_HEADERS,
				DEFAULT_SUPPORTS_CREDENTIALS, DEFAULT_PREFLIGHT_MAXAGE,
				DEFAULT_LOGGING_ENABLED, DEFAULT_DECORATE_REQUEST);

		this.filterConfig = filterConfig;
		this.loggingEnabled = false;

		if (filterConfig != null) {
			String configAllowedOrigins
					= filterConfig.getInitParameter(PARAM_CORS_ALLOWED_ORIGINS);
			String configAllowedHttpMethods
					= filterConfig.getInitParameter(PARAM_CORS_ALLOWED_METHODS);
			String configAllowedHttpHeaders
					= filterConfig.getInitParameter(PARAM_CORS_ALLOWED_HEADERS);
			String configExposedHeaders
					= filterConfig.getInitParameter(PARAM_CORS_EXPOSED_HEADERS);
			String configSupportsCredentials
					= filterConfig
					.getInitParameter(PARAM_CORS_SUPPORT_CREDENTIALS);
			String configPreflightMaxAge
					= filterConfig.getInitParameter(PARAM_CORS_PREFLIGHT_MAXAGE);
			String configLoggingEnabled
					= filterConfig.getInitParameter(PARAM_CORS_LOGGING_ENABLED);
			String configDecorateRequest
					= filterConfig.getInitParameter(PARAM_CORS_REQUEST_DECORATE);

			parseAndStore(configAllowedOrigins, configAllowedHttpMethods,
					configAllowedHttpHeaders,
					configExposedHeaders, configSupportsCredentials,
					configPreflightMaxAge,
					configLoggingEnabled, configDecorateRequest);
		}
	}

	// --------------------------------------------------------------- Handlers
	/**
	 * Handles a CORS request of type {@link CORSRequestType}.SIMPLE.
	 *
	 * @param request The {@link HttpServletRequest} object.
	 * @param response The {@link HttpServletResponse} object.
	 * @param filterChain The {@link FilterChain} object.
	 * @throws IOException ex
	 * @throws ServletException ex
	 * @see <a href="http://www.w3.org/TR/cors/#resource-requests">Simple Cross-Origin Request, Actual Request, and
	 * Redirects</a>
	 */
	public void handleSimpleCORS(final HttpServletRequest request,
			final HttpServletResponse response, final FilterChain filterChain)
			throws IOException, ServletException {
		CORSFilter.CORSRequestType requestType
				= checkRequestType(request);
		if (!(requestType == CORSFilter.CORSRequestType.SIMPLE
				|| requestType == CORSFilter.CORSRequestType.ACTUAL)) {
			String message
					= "Expects a HttpServletRequest object of type "
					+ CORSFilter.CORSRequestType.SIMPLE
					+ " or "
					+ CORSFilter.CORSRequestType.ACTUAL;
			throw new IllegalArgumentException(message);
		}

		final String origin
				= request.getHeader(CORSFilter.REQUEST_HEADER_ORIGIN);
		final String method = request.getMethod();

		// Section 6.1.2
		if (!isOriginAllowed(origin)) {
			handleInvalidCORS(request, response, filterChain);
			return;
		}

		if (!allowedHttpMethods.contains(method)) {
			handleInvalidCORS(request, response, filterChain);
			return;
		}

        // Section 6.1.3
		// Add a single Access-Control-Allow-Origin header.
		if (anyOriginAllowed && !supportsCredentials) {
            // If resource doesn't support credentials and if any origin is
			// allowed
			// to make CORS request, return header with '*'.
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN, "*");
		} else {
            // If the resource supports credentials add a single
			// Access-Control-Allow-Origin header, with the value of the Origin
			// header as value.
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN,
					origin);
		}
        // Section 6.1.3
		// If the resource supports credentials, add a single
		// Access-Control-Allow-Credentials header with the case-sensitive
		// string "true" as value.
		if (supportsCredentials) {
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_CREDENTIALS,
					"true");
		}

        // Section 6.1.4
		// If the list of exposed headers is not empty add one or more
		// Access-Control-Expose-Headers headers, with as values the header
		// field names given in the list of exposed headers.
		if ((exposedHeaders != null) && (!exposedHeaders.isEmpty())) {
			String exposedHeadersString = join(exposedHeaders, ",");
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS,
					exposedHeadersString);
		}

		// Forward the request down the filter chain.
		filterChain.doFilter(request, response);
	}

	/**
	 * Handles CORS pre-flight request.
	 *
	 * @param request The {@link HttpServletRequest} object.
	 * @param response The {@link HttpServletResponse} object.
	 * @param filterChain The {@link FilterChain} object.
	 * @throws IOException ex
	 * @throws ServletException ex
	 */
	public void handlePreflightCORS(final HttpServletRequest request,
			final HttpServletResponse response, final FilterChain filterChain)
			throws IOException, ServletException {
		CORSRequestType requestType = checkRequestType(request);
		if (requestType != CORSRequestType.PRE_FLIGHT) {
			throw new IllegalArgumentException(
					"Expects a HttpServletRequest object of type "
					+ CORSRequestType.PRE_FLIGHT.name().toLowerCase());
		}

		final String origin
				= request.getHeader(CORSFilter.REQUEST_HEADER_ORIGIN);

		// Section 6.2.2
		if (!isOriginAllowed(origin)) {
			handleInvalidCORS(request, response, filterChain);
			return;
		}

		// Section 6.2.3
		String accessControlRequestMethod
				= request.getHeader(CORSFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD);
		if (accessControlRequestMethod == null
				|| (!HTTP_METHODS
				.contains(accessControlRequestMethod.trim()))) {
			handleInvalidCORS(request, response, filterChain);
			return;
		} else {
			accessControlRequestMethod = accessControlRequestMethod.trim();
		}

		// Section 6.2.4
		String accessControlRequestHeadersHeader
				= request.getHeader(CORSFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);
		List<String> accessControlRequestHeaders = new LinkedList<>();
		if (StringUtils.isNotBlank(accessControlRequestHeadersHeader)) {
			String[] headers = accessControlRequestHeadersHeader.trim().split(",");
			for (String header : headers) {
				accessControlRequestHeaders.add(header.trim().toLowerCase());
			}
		}

		// Section 6.2.5
		if (!allowedHttpMethods.contains(accessControlRequestMethod)) {
			handleInvalidCORS(request, response, filterChain);
			return;
		}

		// Section 6.2.6
		for (String header : accessControlRequestHeaders) {
			if (!allowedHttpHeaders.contains(header)) {
				handleInvalidCORS(request, response, filterChain);
				return;
			}
		}

		// Section 6.2.7
		if (supportsCredentials) {
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN,
					origin);
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_CREDENTIALS,
					"true");
		} else {
			if (anyOriginAllowed) {
				response.addHeader(
						CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN,
						"*");
			} else {
				response.addHeader(
						CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN,
						origin);
			}
		}

		// Section 6.2.8
		if (preflightMaxAge > 0) {
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_MAX_AGE,
					String.valueOf(preflightMaxAge));
		}

		// Section 6.2.9
		response.addHeader(
				CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_METHODS,
				accessControlRequestMethod);

		// Section 6.2.10
		if ((allowedHttpHeaders != null) && (!allowedHttpHeaders.isEmpty())) {
			response.addHeader(
					CORSFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_HEADERS,
					join(allowedHttpHeaders, ","));
		}

		// Do not forward the request down the filter chain.
	}

	/**
	 * Handles a request, that's not a CORS request, but is a valid request i.e. it is not a cross-origin request. This
	 * implementation, just forwards the request down the filter chain.
	 *
	 * @param request The {@link HttpServletRequest} object.
	 * @param response The {@link HttpServletResponse} object.
	 * @param filterChain The {@link FilterChain} object.
	 * @throws IOException ex
	 * @throws ServletException ex
	 */
	public void handleNonCORS(final HttpServletRequest request,
			final HttpServletResponse response, final FilterChain filterChain)
			throws IOException, ServletException {
		// Let request pass.
		filterChain.doFilter(request, response);
	}

	/**
	 * Handles a CORS request that violates specification.
	 *
	 * @param request The {@link HttpServletRequest} object.
	 * @param response The {@link HttpServletResponse} object.
	 * @param filterChain The {@link FilterChain} object.
	 */
	public void handleInvalidCORS(final HttpServletRequest request,
			final HttpServletResponse response, final FilterChain filterChain) {
		String origin = request.getHeader(CORSFilter.REQUEST_HEADER_ORIGIN);
		String method = request.getMethod();
		String accessControlRequestHeaders
				= request.getHeader(REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);

		String message
				= "Invalid CORS request; Origin=" + origin + ";Method=" + method;
		if (accessControlRequestHeaders != null) {
			message
					= message + ";Access-Control-Request-Headers="
					+ accessControlRequestHeaders;
		}
		response.setContentType("text/plain");
		response.setStatus(HttpServletResponse.SC_FORBIDDEN);
		response.resetBuffer();

		log(message);
	}

	@Override
	public void destroy() {
		// NOOP
	}

	// -------------------------------------------------------- Utility methods
	/**
	 * Decorates the {@link HttpServletRequest}, with CORS attributes.
	 * <ul>
	 * <li><b>cors.isCorsRequest:</b> Flag to determine if request is a CORS request. Set to <code>true</code> if CORS
	 * request; <code>false</code> otherwise.</li>
	 * <li><b>cors.request.origin:</b> The Origin URL.</li>
	 * <li><b>cors.request.type:</b> Type of request. Values: <code>simple</code> or <code>preflight</code> or
	 * <code>not_cors</code> or <code>invalid_cors</code></li>
	 * <li><b>cors.request.headers:</b> Request headers sent as 'Access-Control-Request-Headers' header, for pre-flight
	 * request.</li>
	 * </ul>
	 *
	 * @param request The {@link HttpServletRequest} object.
	 * @param corsRequestType The {@link CORSRequestType} object.
	 */
	public static void decorateCORSProperties(final HttpServletRequest request,
			final CORSRequestType corsRequestType) {
		if (request == null) {
			throw new IllegalArgumentException(
					"HttpServletRequest object is null");
		}

		if (corsRequestType == null) {
			throw new IllegalArgumentException("CORSRequestType object is null");
		}

		switch (corsRequestType) {
			case SIMPLE:
			case ACTUAL:
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST, true);
				request.setAttribute(CORSFilter.HTTP_REQUEST_ATTRIBUTE_ORIGIN,
						request.getHeader(CORSFilter.REQUEST_HEADER_ORIGIN));
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE,
						corsRequestType.name().toLowerCase());
				break;
			case PRE_FLIGHT:
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST, true);
				request.setAttribute(CORSFilter.HTTP_REQUEST_ATTRIBUTE_ORIGIN,
						request.getHeader(CORSFilter.REQUEST_HEADER_ORIGIN));
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE,
						corsRequestType.name().toLowerCase());
				String headers
						= request.getHeader(REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);
				if (headers == null) {
					headers = "";
				}
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_HEADERS,
						headers
				);
				break;
			case NOT_CORS:
				request.setAttribute(
						CORSFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST, false);
				break;
			default:
				// Don't set any attributes
				break;
		}
	}

	/**
	 * Joins elements of {@link Set} into a string, where each element is separated by the provided separator.
	 *
	 * @param elements The {@link Set} containing elements to join together.
	 * @param joinSeparator The character to be used for separating elements.
	 * @return The joined {@link String}; <code>null</code> if elements {@link Set} is null.
	 */
	public static String join(final Collection<String> elements,
			final String joinSeparator) {
		String separator = ",";
		if (elements == null) {
			return null;
		}
		if (joinSeparator != null) {
			separator = joinSeparator;
		}
		StringBuilder buffer = new StringBuilder();
		boolean isFirst = true;
		for (String element : elements) {
			if (!isFirst) {
				buffer.append(separator);
			} else {
				isFirst = false;
			}

			if (element != null) {
				buffer.append(element);
			}
		}

		return buffer.toString();
	}

	/**
	 * Determines the request type.
	 * @param request req
	 * @return CORS request type
	 */
	public CORSRequestType checkRequestType(final HttpServletRequest request) {
		CORSRequestType requestType = CORSRequestType.INVALID_CORS;
		if (request == null) {
			throw new IllegalArgumentException(
					"HttpServletRequest object is null");
		}
		String originHeader = request.getHeader(REQUEST_HEADER_ORIGIN);
		// Section 6.1.1 and Section 6.2.1
		if (originHeader != null) {
			if (originHeader.isEmpty()) {
				requestType = CORSRequestType.INVALID_CORS;
			} else if (!isValidOrigin(originHeader)) {
				requestType = CORSRequestType.INVALID_CORS;
			} else {
				String method = StringUtils.trimToEmpty(request.getMethod());
				if (HTTP_METHODS.contains(method)) {
					if ("OPTIONS".equals(method)) {
						String accessControlRequestMethodHeader
								= request.getHeader(REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD);
						if (StringUtils.isNotBlank(accessControlRequestMethodHeader)) {
							requestType = CORSRequestType.PRE_FLIGHT;
						} else if (StringUtils.isWhitespace(accessControlRequestMethodHeader)) {
							requestType = CORSRequestType.INVALID_CORS;
						} else {
							requestType = CORSRequestType.ACTUAL;
						}
					} else if ("GET".equals(method) || "HEAD".equals(method)) {
						requestType = CORSRequestType.SIMPLE;
					} else if ("POST".equals(method)) {
						String contentType = request.getContentType();
						if (contentType != null) {
							contentType = contentType.toLowerCase().trim();
							if (SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES
									.contains(contentType)) {
								requestType = CORSRequestType.SIMPLE;
							} else {
								requestType = CORSRequestType.ACTUAL;
							}
						}
					} else if (COMPLEX_HTTP_METHODS.contains(method)) {
						requestType = CORSRequestType.ACTUAL;
					}
				}
			}
		} else {
			requestType = CORSRequestType.NOT_CORS;
		}

		return requestType;
	}

	/**
	 * Checks if the Origin is allowed to make a CORS request.
	 *
	 * @param origin The Origin.
	 * @return <code>true</code> if origin is allowed; <code>false</code> otherwise.
	 */
	private boolean isOriginAllowed(final String origin) {
		if (anyOriginAllowed) {
			return true;
		}

        // If 'Origin' header is a case-sensitive match of any of allowed
		// origins, then return true, else return false.
		return allowedOrigins.contains(origin);
	}

	private void log(String message) {
		if (loggingEnabled) {
			filterConfig.getServletContext().log(message);
		}
	}

	/**
	 * Parses each param-value and populates configuration variables. If a param is provided, it overrides the default.
	 *
	 * @param allowedOrigins A {@link String} of comma separated origins.
	 * @param allowedHttpMethods A {@link String} of comma separated HTTP methods.
	 * @param allowedHttpHeaders A {@link String} of comma separated HTTP headers.
	 * @param exposedHeaders A {@link String} of comma separated headers that needs to be exposed.
	 * @param supportsCredentials "true" if support credentials needs to be enabled.
	 * @param preflightMaxAge The amount of seconds the user agent is allowed to cache the result of the pre-flight
	 * request.
	 * @param loggingEnabled Flag to control logging to access log.
	 * @throws ServletException ex
	 */
	private void parseAndStore(final String allowedOrigins,
			final String allowedHttpMethods, final String allowedHttpHeaders,
			final String exposedHeaders, final String supportsCredentials,
			final String preflightMaxAge, final String loggingEnabled,
			final String decorateRequest)
			throws ServletException {
		if (allowedOrigins != null) {
			if (allowedOrigins.trim().equals("*")) {
				this.anyOriginAllowed = true;
			} else {
				this.anyOriginAllowed = false;
				Set<String> setAllowedOrigins
						= parseStringToSet(allowedOrigins);
				this.allowedOrigins.clear();
				this.allowedOrigins.addAll(setAllowedOrigins);
			}
		}

		if (allowedHttpMethods != null) {
			Set<String> setAllowedHttpMethods
					= parseStringToSet(allowedHttpMethods);
			this.allowedHttpMethods.clear();
			this.allowedHttpMethods.addAll(setAllowedHttpMethods);
		}

		if (allowedHttpHeaders != null) {
			Set<String> setAllowedHttpHeaders
					= parseStringToSet(allowedHttpHeaders);
			Set<String> lowerCaseHeaders = new HashSet<>();
			for (String header : setAllowedHttpHeaders) {
				String lowerCase = header.toLowerCase();
				lowerCaseHeaders.add(lowerCase);
			}
			this.allowedHttpHeaders.clear();
			this.allowedHttpHeaders.addAll(lowerCaseHeaders);
		}

		if (exposedHeaders != null) {
			Set<String> setExposedHeaders = parseStringToSet(exposedHeaders);
			this.exposedHeaders.clear();
			this.exposedHeaders.addAll(setExposedHeaders);
		}

		if (supportsCredentials != null) {
			// For any value other then 'true' this will be false.
			this.supportsCredentials
					= Boolean.parseBoolean(supportsCredentials);
		}

		if (preflightMaxAge != null) {
			try {
				if (!preflightMaxAge.isEmpty()) {
					this.preflightMaxAge = Long.parseLong(preflightMaxAge);
				} else {
					this.preflightMaxAge = 0L;
				}
			} catch (NumberFormatException e) {
				throw new ServletException("Unable to parse preflightMaxAge", e);
			}
		}

		if (loggingEnabled != null) {
			// For any value other then 'true' this will be false.
			this.loggingEnabled = Boolean.parseBoolean(loggingEnabled);
		}

		if (decorateRequest != null) {
			// For any value other then 'true' this will be false.
			this.decorateRequest = Boolean.parseBoolean(decorateRequest);
		}
	}

	/**
	 * Takes a comma separated list and returns a Set<String>.
	 *
	 * @param data A comma separated list of strings.
	 * @return Set<String>
	 */
	private Set<String> parseStringToSet(final String data) {
		String[] splits;

		if (data != null && data.length() > 0) {
			splits = data.split(",");
		} else {
			splits = new String[]{};
		}

		Set<String> set = new HashSet<>();
		if (splits.length > 0) {
			for (String split : splits) {
				set.add(split.trim());
			}
		}

		return set;
	}

	/**
	 * Checks if a given origin is valid or not. Criteria:
	 * <ul>
	 * <li>If an encoded character is present in origin, it's not valid.</li>
	 * <li>Origin should be a valid {@link URI}</li>
	 * </ul>
	 *
	 * @param origin origin
	 * @see <a href="http://tools.ietf.org/html/rfc952">RFC952</a>
	 * @return boolean
	 */
	public static boolean isValidOrigin(String origin) {
		// Checks for encoded characters. Helps prevent CRLF injection.
		if (origin.contains("%")) {
			return false;
		}

		URI originURI;

		try {
			originURI = new URI(origin);
		} catch (URISyntaxException e) {
			return false;
		}
		// If scheme for URI is null, return false. Return true otherwise.
		return originURI.getScheme() != null;

	}

	// -------------------------------------------------------------- Accessors
	/**
	 * Determines if logging is enabled or not.
	 *
	 * @return <code>true</code> if it's enabled; false otherwise.
	 */
	public boolean isLoggingEnabled() {
		return loggingEnabled;
	}

	/**
	 * Determines if any origin is allowed to make CORS request.
	 *
	 * @return <code>true</code> if it's enabled; false otherwise.
	 */
	public boolean isAnyOriginAllowed() {
		return anyOriginAllowed;
	}

	/**
	 * Returns a {@link Set} of headers that should be exposed by browser.
	 *
	 * @return list
	 */
	public Collection<String> getExposedHeaders() {
		return exposedHeaders;
	}

	/**
	 * Determines is supports credentials is enabled.
	 *
	 * @return boolean
	 */
	public boolean isSupportsCredentials() {
		return supportsCredentials;
	}

	/**
	 * Returns the preflight response cache time in seconds.
	 *
	 * @return Time to cache in seconds.
	 */
	public long getPreflightMaxAge() {
		return preflightMaxAge;
	}

	/**
	 * Returns the {@link Set} of allowed origins that are allowed to make requests.
	 *
	 * @return {@link Set}
	 */
	public Collection<String> getAllowedOrigins() {
		return allowedOrigins;
	}

	/**
	 * Returns a {@link Set} of HTTP methods that are allowed to make requests.
	 *
	 * @return {@link Set}
	 */
	public Collection<String> getAllowedHttpMethods() {
		return allowedHttpMethods;
	}

	/**
	 * Returns a {@link Set} of headers support by resource.
	 *
	 * @return {@link Set}
	 */
	public Collection<String> getAllowedHttpHeaders() {
		return allowedHttpHeaders;
	}

	// -------------------------------------------------- CORS Response Headers
	/**
	 * The Access-Control-Allow-Origin header indicates whether a resource can be shared based by returning the value of
	 * the Origin request header in the response.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN
			= "Access-Control-Allow-Origin";

	/**
	 * The Access-Control-Allow-Credentials header indicates whether the response to request can be exposed when the
	 * omit credentials flag is unset. When part of the response to a preflight request it indicates that the actual
	 * request can include user credentials.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_CREDENTIALS
			= "Access-Control-Allow-Credentials";

	/**
	 * The Access-Control-Expose-Headers header indicates which headers are safe to expose to the API of a CORS API
	 * specification.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS
			= "Access-Control-Expose-Headers";

	/**
	 * The Access-Control-Max-Age header indicates how long the results of a preflight request can be cached in a
	 * preflight result cache.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_MAX_AGE
			= "Access-Control-Max-Age";

	/**
	 * The Access-Control-Allow-Methods header indicates, as part of the response to a preflight request, which methods
	 * can be used during the actual request.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_METHODS
			= "Access-Control-Allow-Methods";

	/**
	 * The Access-Control-Allow-Headers header indicates, as part of the response to a preflight request, which header
	 * field names can be used during the actual request.
	 */
	public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_HEADERS
			= "Access-Control-Allow-Headers";

	// -------------------------------------------------- CORS Request Headers
	/**
	 * The Origin header indicates where the cross-origin request or preflight request originates from.
	 */
	public static final String REQUEST_HEADER_ORIGIN = "Origin";

	/**
	 * The Access-Control-Request-Method header indicates which method will be used in the actual request as part of the
	 * preflight request.
	 */
	public static final String REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD
			= "Access-Control-Request-Method";

	/**
	 * The Access-Control-Request-Headers header indicates which headers will be used in the actual request as part of
	 * the preflight request.
	 */
	public static final String REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS
			= "Access-Control-Request-Headers";

	// ----------------------------------------------------- Request attributes
	/**
	 * The prefix to a CORS request attribute.
	 */
	public static final String HTTP_REQUEST_ATTRIBUTE_PREFIX = "cors.";

	/**
	 * Attribute that contains the origin of the request.
	 */
	public static final String HTTP_REQUEST_ATTRIBUTE_ORIGIN
			= HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.origin";

	/**
	 * Boolean value, suggesting if the request is a CORS request or not.
	 */
	public static final String HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST
			= HTTP_REQUEST_ATTRIBUTE_PREFIX + "isCorsRequest";

	/**
	 * Type of CORS request, of type {@link CORSRequestType}.
	 */
	public static final String HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE
			= HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.type";

	/**
	 * Request headers sent as 'Access-Control-Request-Headers' header, for pre-flight request.
	 */
	public static final String HTTP_REQUEST_ATTRIBUTE_REQUEST_HEADERS
			= HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.headers";

	// -------------------------------------------------------------- Constants
	/**
	 * Enumerates varies types of CORS requests. Also, provides utility methods to determine the request type.
	 */
	public enum CORSRequestType {

		/**
		 * A simple HTTP request, i.e. it shouldn't be pre-flighted.
		 */
		SIMPLE,
		/**
		 * A HTTP request that needs to be pre-flighted.
		 */
		ACTUAL,
		/**
		 * A pre-flight CORS request, to get meta information, before a non-simple HTTP request is sent.
		 */
		PRE_FLIGHT,
		/**
		 * Not a CORS request, but a normal request.
		 */
		NOT_CORS,
		/**
		 * An invalid CORS request, i.e. it qualifies to be a CORS request, but fails to be a valid one.
		 */
		INVALID_CORS
	}

	/**
	 * {@link Collection} of HTTP methods. Case sensitive.
	 *
	 * see http://tools.ietf.org/html/rfc2616#section-5.1.1
	 */
	public static final Collection<String> HTTP_METHODS = new HashSet<String>(
			Arrays.asList("OPTIONS", "GET", "HEAD", "POST", "PUT", "PATCH", "DELETE",
					"TRACE", "CONNECT"));
	/**
	 * {@link Collection} of non-simple HTTP methods. Case sensitive.
	 */
	public static final Collection<String> COMPLEX_HTTP_METHODS
			= new HashSet<String>(
					Arrays.asList("PUT", "PATCH", "DELETE", "TRACE", "CONNECT"));
	/**
	 * {@link Collection} of Simple HTTP methods. Case sensitive.
	 *
	 * see http://www.w3.org/TR/cors/#terminology
	 */
	public static final Collection<String> SIMPLE_HTTP_METHODS
			= new HashSet<String>(
					Arrays.asList("GET", "POST", "HEAD"));

	/**
	 * {@link Collection} of Simple HTTP request headers. Case in-sensitive.
	 *
	 * see http://www.w3.org/TR/cors/#terminology
	 */
	public static final Collection<String> SIMPLE_HTTP_REQUEST_HEADERS
			= new HashSet<String>(Arrays.asList("Accept", "Accept-Language",
							"Content-Language"));

	/**
	 * {@link Collection} of Simple HTTP request headers. Case in-sensitive.
	 *
	 * see http://www.w3.org/TR/cors/#terminology
	 */
	public static final Collection<String> SIMPLE_HTTP_RESPONSE_HEADERS
			= new HashSet<String>(Arrays.asList("Cache-Control",
							"Content-Language", "Content-Type", "Expires",
							"Last-Modified", "Pragma"));

	/**
	 * {@link Collection} of Simple HTTP request headers. Case in-sensitive.
	 *
	 * see http://www.w3.org/TR/cors/#terminology
	 */
	public static final Collection<String> SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES
			= new HashSet<String>(Arrays.asList(
							"application/x-www-form-urlencoded", "multipart/form-data",
							"text/plain"));

	// ------------------------------------------------ Configuration Defaults
	/**
	 * By default, all origins are allowed to make requests.
	 */
	public static final String DEFAULT_ALLOWED_ORIGINS = "*";

	/**
	 * By default, following methods are supported: GET, POST, HEAD and OPTIONS.
	 */
	public static final String DEFAULT_ALLOWED_HTTP_METHODS
			= "GET,POST,HEAD,OPTIONS";

	/**
	 * By default, time duration to cache pre-flight response is 30 mins.
	 */
	public static final String DEFAULT_PREFLIGHT_MAXAGE = "1800";

	/**
	 * By default, support credentials is turned on.
	 */
	public static final String DEFAULT_SUPPORTS_CREDENTIALS = "true";

	/**
	 * By default, following headers are supported: Origin,Accept,X-Requested-With, Content-Type,
	 * Access-Control-Request-Method, and Access-Control-Request-Headers.
	 */
	public static final String DEFAULT_ALLOWED_HTTP_HEADERS
			= "Origin,Accept,X-Requested-With,Content-Type,"
			+ "Access-Control-Request-Method,Access-Control-Request-Headers";

	/**
	 * By default, none of the headers are exposed in response.
	 */
	public static final String DEFAULT_EXPOSED_HEADERS = "";

	/**
	 * By default, access log logging is turned off.
	 */
	public static final String DEFAULT_LOGGING_ENABLED = "false";

	/**
	 * By default, request is decorated with CORS attributes.
	 */
	public static final String DEFAULT_DECORATE_REQUEST = "true";

	// ----------------------------------------Filter Config Init param-name(s)
	/**
	 * Key to retrieve allowed origins from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_ALLOWED_ORIGINS
			= "cors.allowed.origins";

	/**
	 * Key to retrieve support credentials from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_SUPPORT_CREDENTIALS
			= "cors.support.credentials";

	/**
	 * Key to retrieve exposed headers from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_EXPOSED_HEADERS
			= "cors.exposed.headers";

	/**
	 * Key to retrieve allowed headers from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_ALLOWED_HEADERS
			= "cors.allowed.headers";

	/**
	 * Key to retrieve allowed methods from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_ALLOWED_METHODS
			= "cors.allowed.methods";

	/**
	 * Key to retrieve preflight max age from {@link FilterConfig}.
	 */
	public static final String PARAM_CORS_PREFLIGHT_MAXAGE
			= "cors.preflight.maxage";

	/**
	 * Key to retrieve access log logging flag.
	 */
	public static final String PARAM_CORS_LOGGING_ENABLED
			= "cors.logging.enabled";

	/**
	 * Key to determine if request should be decorated.
	 */
	public static final String PARAM_CORS_REQUEST_DECORATE
			= "cors.request.decorate";
}
